1. The Field of the Invention
The field of the invention relates to an improved screw fastener blank and a method of manufacture therefor. The blank has a head and shank portion of corrosion resistant material to which is adhered and made integral therewith a drill slug portion of a different material.
2. Description of the Prior Art
U.S. Pat. Nos. 751,258 to Carstens (Feb. 2, 1904); 3,180,126 to Carlson (Apr. 27, 1965) and 3,978,760 to Muenchinger (Sept. 7, 1978) which teach self-tapping fasteners; U.S. Pat. Nos. 3,204,516 to Weiber (Sept. 7, 1965) and 3,207,024 to Sommer (Sept. 21, 1965) which teach fabrication of drill points on screw ends; U.S. Pat. Nos. 3,585,894 to Brown (June 22, 1971); 4,257,307 to Regensberger (Mar. 24, 1981) and 4,480,951 to Regensberger (Nov. 6, 1984) which teach fabrication of self-drilling and tapping screw fasteners; U.S. Pat. Nos. 1,765,516 to Whitner (June 24, 1930); 3,611,862 to Walker (Oct. 12, 1971) and 4,003,175 to Patry (Jan. 18, 1977) which teach the use of bi-compositional fasteners, connectors and parts generally and U.S. Pat. Nos. 910,434 to Thompson (Jan. 19, 1909) and 2,153,702 to Tighe (Apr. 11, 1939) which teaches fabrication of bi-metallic screw fasteners and U.S. Pat. No. 3,924,508 to DeCaro (Dec. 9, 1975) which teaches a bi-metallic drill screw.
The desirability of a bi-compositional screw fastener which is self-drilling and tapping has long been apparent. Such a fastener would embody the properties of each material where it is used. For example, the head and shank portions may be exposed to heat or a corrosive atmosphere, or require a desired finish which is exposed to view while the drill and unexposed portions might require hardness, thread forming capability or holding power but not require corrosion resistance or beauty.
Joinder of components of dissimilar materials, metals or alloys pose problems inherent in their differences. Thompson (U.S. Pat. No. 910,434) discusses the butt welding of the screw portion of a screw blank to be composed of, for example, brass to steel. U.S. Pat. No. 910,434 teaches that different conductivities can be overcome by reducing the contact surface area of the material with the higher conductivity inverse proportionately to the welding area of the material with the lower conductivity so that the heat of resistance welding will rise relatively more quickly in the part with poorer conductivity thereby effecting a sound weld. In addition to being inaccurate as to conductivity vs. surface area, such a process will inevitably create a flash of excess weld material in the vicinity of the weld interface, see U.S. Pat. No. 910,434, FIG. 2, which must be eliminated by grinding. This problem is not solved by Tighe (U.S. Pat. No. 2,153,702), p. 2, col 2, lines 13 and 14. See also, FIG. 3 of the present invention. The flash may be disposed of without grinding if the screw blank is cut with threads. In thread rolling such excesss is undesirable.
DeCarlo (U.S. Pat. No. 3,924,508) teaches a method of joinder which uses inertial or friction welding to overcome the problem of different conductivities. The process disclosed requires specialized equipment in manufacture to impart an extremely high speed rotational component to the part to be joined and one component part must have a formed recess in the fixed part to receive the welding flash.
The present invention accomplishes joinder with conventional butt welding equipment and leaves a blank suitable for thread rolling.